Accurately detecting the speed of rotating equipment is critical for the measurement of vibration. A precise speed measurement is required to differentiate faults relating to the machine's synchronous running speed, such as imbalance and misalignment to sub-synchronous frequencies such as belt frequencies to those of non-synchronous frequencies, such as bearing faults and electrical frequencies. A precise phase reference is required to perform synchronous analysis and measure relative phase. While some machines are equipped with permanently mounted sensors that produce a tachometer pulse, others must be measured in-situ by an operator. For an accurate phase reference, both methods require an accurate detection of a feature on the rotating shaft that is exposed once per rotation. An in-situ measurement of speed can be performed using a contact tachometer, a stroboscope, a photo tachometer or other means, such as a CSI Model 430 speed sensor. All of these in-situ measurement devices have their disadvantages.
A contact tachometer performs a direct measurement involving physical contact with the surface or end of a rotating shaft. This requires the operator to be in close proximity to the machine.
A stroboscope flashes high-intensity light at a variable rate while the operator adjusts the flash rate to “freeze” the motion of the shaft to determine the operating speed. Some level of operator skill is necessary to interpret the illuminated scene correctly.
A photo tachometer uses light reflected from a special reflective tape attached to the shaft to create a once-per-revolution pulse. This requires the machine to be stopped to apply or clean the reflective tape.
The CSI Model 430 speed sensor uses reflected laser light to detect surface irregularities of a rotating part. Speed information is automatically extracted from the reflection signal by autocorrelation. This method does not require special tape or shaft preparation and the measurement can be made at quite some distance. However, in order to generate an accurate once-per-revolution (1×) tachometer pulse for phase reference, the laser beam must be precisely aimed at a unique once-per-revolution feature, such as a keyway, reflective tape, flat or set screw.
Often in an industrial environment, acquiring an accurate rotational speed measurement, and in particular acquiring an accurate phase measurement, requires both a tachometer and a strobe light. A tachometer pulse (such as produced from a laser tachometer) is generated when an object such as a keyway, reflective tape, or indention in the shaft is present. However, such protrusions in an otherwise smooth shaft are impossible to see with the naked eye when the machine is running, even under bright lights. Generally, the only way to find a mark on a rotating shaft is with a strobe light. With manual adjustments of the strobe flash rate, a speed can be determined, although no pulse can be generated for determining the phase relationship. However, by using the strobe to find distinct shaft perturbations, the laser can be used to precisely determine phase relationships.
What is needed, therefore, is a rotational speed detection device that combines the functions of a strobe light with the functions of a laser tachometer.